Safe storage and transportation of sulfur dioxide

ABSTRACT

The invention relates to a method of confining sulfur dioxide for storage or transportation under safe conditions. The method of the invention comprises the steps of (a) contacting a sulfur dioxide-containing gas stream with an absorbing medium comprising water and a water-soluble amine absorbent having at least one amine group with a pKa value greater than about 7 and at least one other amine group with a pKa value less than about 6.5 so that the at least one amine group with a pKa value greater than about 7 irreversibly absorbs sulfur dioxide in salt form rendering the amine absorbent non-volatile and the at least one other amine group with a pKa value less than about 6.5 reversibly absorbs sulfur dioxide, to thereby saturate the absorbing medium with sulfur dioxide against a partial pressure of sulfur dioxide of no more than about 1 atmosphere at 25° C.; and (b) charging the absorbing medium saturated with sulfur dioxide obtained in step (a) into a container for storage or transportation.

This application is a 371 of PCT/CA98/00237, filed Mar. 16, 1998.

TECHNICAL FIELD

The present invention pertains to improvements in the field of storageand transportation of sulfur dioxide. More particularly, the inventionrelates to a method of confining sulfur dioxide for storage and/ortransportation under safe conditions.

BACKGROUND ART

Sulfur dioxide (SO₂) is a widely used chemical in industries such aswood pulping and bleaching, corn wet milling, water treatment and theproduction of sulfuric acid. It is a colorless, nonflammable gas with aboiling point of −10° C. at atmospheric pressure. Sulfur dioxide ishighly toxic by inhalation and a strong irritant to the eyes and mucousmembranes. It is also a dangerous air contaminant and constituent ofsmog.

Currently, bulk quantities of sulfur dioxide are stored and transportedas a liquid in suitable pressure vessels. The vapor pressure of liquidsulfur dioxide at temperatures that may occur in normal storage andtransport operations can be up to 8 bar. Thus, in case of a leak in orrupture of the pressure vessel used to store or transport liquid sulfurdioxide, particularly if the damage occurs towards the bottom of thevessel, large quantities of sulfur dioxide can be released from the tankvery rapidly. Since the sulfur dioxide at ambient temperature is aboveits boiling point, any liquid sulfur dioxide released to the atmospherewill vaporize rapidly, creating a vapor cloud of toxic gas that tends tostay at ground level, being heavier than air. Prevailing winds can thendisperse the vapors, creating conditions hazardous to health or evenlethal conditions over a large area. Concentration of 5 to 10 ppmv ofsulfur dioxide in air will lead to irritation of the respiratory tractand concentrations above 400 to 500 parts per million by volume (ppmv),even for a few minutes, are dangerous to life. Areas adjacent toindustrial SO₂ storage sites and railroads or roads used for thetransportation of sulfur dioxide are thus at risk in the event of arelease.

While it is known that sulfur dioxide dissolves in water to the extentof about 10% by weight, it is not a desirable solvent for the purpose ofstoring or transporting sulfur dioxide because of the expense ofproviding large tanks for the dilute solution. Moreover, it is noteconomical to transport sulfur dioxide in such a water solution becauseof the excessive cost of transporting nine tons of water for each ton ofSO₂. Some organic compounds such as chloroform, formic acid, aceticacid, methanol, ethanol and acetone have high solvent power for sulfurdioxide, but these have the disadvantage that they are volatile andwould contaminate the regenerated sulfur dioxide with undesirableimpurities. In addition, most of these compounds are flammable, thuspresenting a fire hazard where none existed before.

Aqueous solutions of alkalis such as sodium hydroxide can dissolvesubstantial quantities of sulfur dioxide by formation of sodium sulfite(Na₂SO₃), sodium bisulfite (NaHSO₃) and sodium pyrosulfite (Na₂S₂O₅).However, regenerability of sulfur dioxide from these solutions isincomplete, the maximum being about 13% by weight (theoretical) from asaturated aqueous solution of the pyrosulfite. This again implies a veryhigh effective transportation cost for the sulfur dioxide.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to overcome the abovedrawbacks and to provide a method of confining sulfur dioxide forstorage and/or transportation under safe conditions.

In accordance with the present invention, there is thus provided amethod of confining sulfur dioxide for storage or transportation undersafe conditions, which comprises the steps of:

(a) contacting a sulfur dioxide-containing gas stream with an absorbingmedium comprising water and a water-soluble amine absorbent having atleast one amine group with a pKa value greater than about 7 and at leastone other amine group with a pKa value less than about 6.5 so that theat least one amine group with a pKa value greater than about 7irreversibly absorbs sulfur dioxide in salt form rendering the amineabsorbent non-volatile and the at least one other amine group with a pKavalue less than about 6.5 reversibly absorbs sulfur dioxide, to therebysaturate the absorbing medium with sulfur dioxide against a partialpressure of sulfur dioxide of no more than about 1 atmosphere at 25° C.;and

(b) charging the absorbing medium saturated with sulfur dioxide obtainedin step (a) into storage or transportation means.

The expression “safe conditions” as used herein refers to conditionspresenting a greatly reduced hazard to life and the environment in thecase of a leak in or rupture of the storage or transportation container,in comparison to a similar leak or rupture when storing or transportingliquid sulfur dioxide. Since the absorbing medium saturated with sulfurdioxide is below its bubble point, the sulfur dioxide vapor cloudgenerated by a leak or spill of such a saturated absorbing medium isrelatively small. With liquid sulfur dioxide, a very large vapor cloudis formed rapidly since essentially all the sulfur dioxide vaporizes.The use of an amine absorbent having at least one amine group with a pKavalue greater than about 7 ensures that the amine absorbent isnonvolatile since such an amine group irreversibly absorbs sulfurdioxide to form a salt which is not regenerable under the normaloperating conditions of the process.

Preferably, the amine absorbent has at least one amine group with a pKavalue of about 7.5 to about 10 and at least one other amine group with apKa value of about 4.5 to about 6.0.

Examples of suitable amine absorbents which may be used in accordancewith the present invention are diamines having the general formula:

wherein R₁ is an alkylene group having 1 to 3 carbon atoms, R₂, R₃, R₄and R₅ are the same or different and each represent a hydrogen atom, alower alkyl group having 1 to 8 carbon atoms or a lower hydroxy-alkylgroup having 2 to 8 carbon atoms, or any of R₂, R₃, R₄ and R₅ formtogether with the nitrogen atoms to which they are attached a 6-memberedring.

Examples of preferred diamines in free base form include:

N,N′,N′-(trimethyl)-N-(2-hydroxyethyl)-ethylenediamine,

N,N,N′,N′-tetramethyl-ethylenediamine,

N,N,N′,N′-tetramethyl-diaminomethane,

N,N,N′,N′-tetrakis-(2-hydroxyethyl)-ethylenediamine,

N,N′-dimethylpiperazine,

N,N,N′,N′-tetrakis-(2-hydroxyethyl)-1,3-diaminopropane,

N′,N′-dimethyl-N,N-bis-(2-hydroxyethyl)-ethylenediamine,

N-methyl N′-(2-hydroxyethyl)-piperazine,

N-(2-hydroxyethyl)-piperazine,

N,N′-bis(2-hydroxyethyl)-piperazine,

N-methyl-piperazine, and piperazine.

According to a preferred embodiment, step (a) is carried out in agas/liquid contact apparatus providing countercurrent gas and liquidflows.

Where the sulfur dioxide-containing gas stream is a gaseous stream ofsubstantially pure water-saturated sulfur dioxide, step (a) ispreferably carried out under substantially atmospheric pressure andambient temperature conditions. On the other hand, when the sulfurdioxide-containing gas stream contains less than about 90% by volume ofsulfur dioxide, step (a) is preferably carried out by:

i) contacting the sulfur dioxide-containing gas stream with theabsorbing medium to produce a sulfur dioxide-laden absorbing medium;

ii) dividing the sulfur dioxide-laden absorbing medium into separatefirst and second portions each representing a predetermined proportionof the sulfur dioxide-laden absorbing medium;

iii) removing the absorbed sulfur dioxide from the second portion ofsulfur dioxide-laden absorbing medium to regenerate the amine absorbentcontained therein and thereby produce a sulfur dioxide-depletedabsorbing medium and a gaseous stream of substantially purewater-saturated sulfur dioxide; and

iv) contacting the gaseous stream of substantially pure water-saturatedsulfur dioxide with the first portion of sulfur dioxide-laden absorbingmedium, whereby the proportion of sulfur dioxide-laden absorbing mediumrepresented by the second portion is such to produce in step (iii)sufficient gaseous sulfur dioxide to saturate in step (iv) the firstportion of sulfur dioxide-laden absorbing medium with sulfur dioxideagainst a partial pressure of sulfur dioxide of no more than about 1atmosphere at 25° C.

The sulfur dioxide-depleted absorbing medium produced in step (a) (iii)is advantageously recycled in step (a) (i) for absorption of sulfurdioxide. Any sulfur dioxide emissions produced in step (a) (iv) arepreferably recycled to step (a) (i) for admixture with the sulfurdioxide-containing gas stream.

When it is desired to use the sulfur dioxide at a consuming site, theabsorbing medium saturated with sulfur dioxide is charged into atransport container or pipeline and conveyed to the consuming site wherethe absorbed sulfur dioxide is removed from the saturated absorbingmedium to regenerate the amine absorbent contained therein and therebyproduce another sulfur dioxide-depleted absorbing medium and a gaseousstream of substantially pure water-saturated sulfur dioxide forconsumption. Preferably, the other sulfur dioxide-depleted absorbingmedium is combined with the sulfur dioxide-depleted absorbing mediumproduced in step (a) (iii) and the combined sulfur dioxide-depletedabsorbing media are recycled to step (a) (i) for absorption of sulfurdioxide.

According to another preferred embodiment where the sulfurdioxide-containing gas stream contains less than about 90% by volume ofsulfur dioxide, step (a) is carried out by:

i) contacting the sulfur dioxide-containing gas stream with a firstabsorbing medium comprising water and the amine absorbent to produce asulfur dioxide-laden absorbing medium;

ii) removing the absorbed sulfur dioxide from the sulfur dioxide-ladenabsorbing medium to regenerate the amine absorbent contained therein andthereby produce a sulfur dioxide-depleted absorbing medium and a gaseousstream of substantially pure water-saturated sulfur dioxide; and

iii) contacting the gaseous stream of substantially pure water-saturatedsulfur dioxide with a second absorbing medium comprising water and theamine absorbent, the first and second absorbing media differing from oneanother in water content or type of amine absorbent so that the secondabsorbing medium has an absorption capacity for sulfur dioxide greaterthan the first absorbing medium, to saturate the second absorbing mediumwith sulfur dioxide against a partial pressure of sulfur dioxide of nomore than about 1 atmosphere at 25° C.

The sulfur dioxide-depleted absorbing medium produced in step (a) (ii)is advantageously recycled in step (a) (i) for absorption of sulfurdioxide. Any sulfur dioxide emissions produced in step (a) (iii) arepreferably recycled to step (a) (i) for admixture with the sulfurdioxide containing gas stream.

According to a further preferred embodiment where the sulfurdioxide-containing gas stream contains less than about 90% by volume ofsulfur dioxide, step (a) is carried out by:

i) contacting the sulfur dioxide-containing gas stream with a firstabsorbing medium comprising water and the amine absorbent to produce afirst sulfur dioxide-laden absorbing medium and a partially scrubbedsulfur dioxide-containing gas stream;

ii) contacting the partially scrubbed sulfur dioxide-containing gasstream with a second absorbing medium comprising water and the amineabsorbent, the first and second absorbing media differing from oneanother in water content or type of amine so that the second absorbingmedium has an absorption capacity for sulfur dioxide less than the firstabsorbing medium, to produce a second sulfur dioxide-laden absorbingmedium;

iii) removing the absorbed sulfur dioxide from the second sulfurdioxide-laden absorbing medium to regenerate the amine absorbentcontained therein and thereby produce a sulfur dioxide-depletedabsorbing medium and a gaseous stream of substantially purewater-saturated sulfur dioxide; and

iv) contacting the gaseous stream of substantially pure water-saturatedsulfur dioxide with the first sulfur dioxide-laden absorbing medium tosaturate the first absorbing medium with sulfur dioxide against apartial pressure of sulfur dioxide of no more than about 1 atmosphere at25° C.

The sulfur dioxide-depleted absorbing medium produced in step (a) (iii)is advantageously recycled to step (a) (ii) for absorption of sulfurdioxide. Any sulfur dioxide emissions produced in step (a) (iv) arepreferably recycled to step (a) (i) for admixture with said sulfurdioxide-containing gas stream.

Preferably, steps (a) (i) and (a) (ii) are carried out in a gas/liquidcontact apparatus comprising first and second gas/liquid contact zonesin gas flow communication with one another. The sulfurdioxide-containing gas stream is contacted in the first zone with thefirst absorbing medium, the partially scrubbed sulfur dioxide-containinggas flowing from the first zone to the second zone for contact with thesecond absorbing medium in the second zone. The sulfur dioxide-depletedabsorbing medium produced in step (a) (iii) is advantageously recycledto step (a) (ii) for absorption of sulfur dioxide in the second zone.Any sulfur dioxide emissions produced in step (a) (iv) are preferablyrecycled to step (a) (i) for admixture with the sulfurdioxide-containing gas stream.

BRIEF DESCRIPTION OF DRAWINGS

Further features and advantages of the invention will become morereadily apparent from the following description of preferred embodimentsthereof, with reference to the accompanying drawings, in which:

FIG. 1 is a flow diagram of a method according to a first preferredembodiment of the invention;

FIG. 2 is another flow diagram of a method according to a secondpreferred embodiment of the invention;

FIG. 3 is a further flow diagram of a method according to a thirdpreferred embodiment of the invention; and

FIG. 4 schematically illustrates how the sulfur dioxide is removed fromthe SO₂-saturated absorbing medium obtained by the methods in accordancewith the flow diagrams of FIGS. 1, 2 and 3.

MODES OF CARRYING OUT THE INVENTION

Referring first to FIG. 1, there is schematically illustrated agas-liquid contact apparatus 10 providing countercurrent gas and liquidflows. A sulfur dioxide-containing gas stream is fed via line 12 to theapparatus 10 wherein it is contacted with an aqueous absorbing mediumcomprising water and a water-soluble diamine absorbent having a firstamine group with a pKa value greater than 7 and a second amine groupwith a pKa value less than 6.5. The first amine group irreversiblyabsorbs sulfur dioxide in salt form rendering the diamine absorbentnon-volatile and the second amine group reversibly absorbs sulfurdioxide. Where the sulfur dioxide-containing gas stream in line 12 is agaseous stream of substantially pure sulfur dioxide, the absorption inthe apparatus 10 is carried out under substantially atmospheric pressureand ambient temperature conditions to saturate the absorbing medium withsulfur dioxide against a partial pressure of sulfur dioxide of no morethan about 1 atmosphere at 25° C. Feed gas residue is discharged fromthe apparatus 10 and passed via line 14 to a stack (not shown). TheSO₂-saturated absorbing medium is withdrawn from the apparatus 10 andpassed via lines 16, 18, 20 and 22 to a buffer storage container 24,valves 26 and 28 being positioned to provide liquid flow communicationbetween lines 16, 18 and 20.

When the sulfur dioxide-containing gas stream in line 12 contains lessthan about 90% by volume of sulfur dioxide, the absorption carried outin the apparatus 10 produces a sulfur-laden absorbing medium which isremoved from the apparatus by line 16 and divided by means of thethree-way valve 26 into separate first and second portions eachrepresenting a predetermined proportions of the sulfur dioxide-ladenabsorbing medium. The first portion is passed via lines 18 and 30 to agas-liquid contact apparatus 32, the two-way valve 28 being positionedto provide liquid flow communication between lines 18 and 30. The secondportion of sulfur dioxide-laden absorbing medium is sent via line 34 toa stripping column 36 for removing the absorbed sulfur dioxide from thesecond portion and regenerating the diamine absorbent contained therein,by means of steam fed from a reboiler 38 via line 40. Steam isintroduced into the reboiler 38 by line 42 and condensate is removedtherefrom by line 44. The sulfur dioxide-depleted absorbing medium isrecovered from the stripping column 36 by line 46, passed through thereboiler 38 and recycled via line 48 of the gas-liquid contact apparatus10 for absorption of sulfur dioxide. A gaseous stream of substantiallypure water-saturated sulfur dioxide is withdrawn from the strippingcolumn 36 into an overhead reflux condenser (not shown) and passed vialine 50 to the gas-liquid contact apparatus 32 wherein it is contactedwith the first portion of sulfur dioxide-laden absorbing medium fed vialine 30.

The proportion of sulfur dioxide-laden absorbing medium represented bythe second portion is such to produce in the stripping column 36sufficient gaseous sulfur dioxide to saturate in the apparatus 32 thefirst portion of sulfur dioxide-laden absorbing with sulfur dioxideagainst a partial pressure of sulfur dioxide of no more than about 1atmosphere at 25° C. The SO₂-saturated absorbing medium is withdrawnfrom the apparatus 32 and passed via line 22 to the buffer storagecontainer 24. Any sulfur dioxide emissions produced in the apparatus 32are removed therefrom and recycled via lines 52 and 12 to the apparatus10, in admixture with the sulfur dioxide-containing gas stream in line12.

The SO₂-saturated absorbing medium in the storage container 24 can betransferred via line 54 to a transport container (not shown) fortransportation to a consuming site where the absorbed sulfur dioxide isremoved from the SO₂-saturated absorbing medium to regenerate thediamine absorbent contained therein and thereby produce another sulfurdioxide-depleted absorbing medium and a gaseous stream of substantiallypure water-saturated sulfur dioxide for consumption. For example, usecan be made at the consuming site of the stripping column 56 illustratedin FIG. 4.

The embodiment illustrated in FIG. 1 has the advantage of ensuring thatmaximum quantities of sulfur dioxide are contained in the absorbingmedium to be stored or shipped, regardless of the SO₂ content in thefeed gas stream, temperature of the absorption or the gas/liquid ratioin the apparatus 10.

As shown in FIG. 4, the SO₂-saturated absorbing medium is fed via line58 to the stripping column 56 wherein the absorbed sulfur dioxide isremoved from the SO₂-saturated absorbing medium by means of steam fedfrom a reboiler 60 via line 62. Steam is introduced into the reboiler 60by line 64 and condensate is removed therefrom by line 66. The gaseousstream of substantially pure water-saturated sulfur dioxide forconsumption is discharged via line 68 from the stripping column 56. Thesulfur dioxide-depleted absorbing medium is recovered from the strippingcolumn 56, passed through the reboiler 60 and charged via line 70 into atransport container (not shown) for recycling to the apparatus 10 shownin FIG. 1. Such a sulfur dioxide-depleted absorbing medium can be eitheradmixed via line 72 with the sulfur dioxide-depleted absorbing medium inline 48 or fed via line 74 directly to the apparatus 10. In the lattercase, the sulfur dioxide-depleted absorbing medium recovered from thestripping column 56 and fed via line 74 mixes with the sulfurdioxide-depleted absorbing medium recovered from the stripping column 36and fed via line 48, in a mixing zone adjacent the inlet through whichthe sulfur dioxide-depleted absorbing medium recovered from thestripping column 56 is fed. This ensures that the degree of SO₂ removalfrom the feed gas stream is controlled to the desired level bycontrolling the degree of diamine regeneration in the stripping column36, regardless of the degree of diamine regeneration in the strippingcolumn 56.

A further embodiment resides in the use of two different absorbingmedia, one being optimized for efficiency and stability for removal ofsulfur dioxide from the feed gas stream, and the other being moredesirable as the storage and/or transport medium. The two absorbingmedia may differ in water content and/or type of amine absorbent, whilestill being of the class of amines described herein. Such an embodimentmay be practiced according to two variants illustrated in FIGS. 2 and 3.

As shown in FIG. 2, a sulfur dioxide-containing gas stream containingless than about 90% by volume of sulfur dioxide is fed via line 76 to agas-liquid contact apparatus 78 wherein it is contacted with a firstabsorbing medium comprising water and a diamine absorbent of the classdescribed herein, to produce a sulfur dioxide-laden absorbing medium.Feed gas residue is discharged from the apparatus 78 and passed via line80 to a stack (not shown). The sulfur dioxide-laden absorbing medium iswithdrawn from the apparatus 78 and passed via line 82 to a strippingcolumn 84 for removing the absorbed sulfur dioxide from the firstabsorbing medium and regenerating the diamine absorbent contained in thefirst absorbing medium, by means of steam fed from a reboiler 86 vialine 88. Steam is introduced into the reboiler 86 by line 90 andcondensate is removed therefrom by line 92. The sulfur dioxide-depletedabsorbing medium is recovered from the stripping column 84 by line 94,passed through the reboiler and recycled via line 96 to the apparatus 78for absorption of sulfur dioxide. A gaseous stream of substantially purewater saturated sulfur dioxide is withdrawn from the stripping column 84into an overhead reflux condenser (not shown) and passed via line 98 toa gas-liquid contact apparatus 100 wherein it is contacted with a secondabsorbing medium comprising water and a diamine absorbent of the classdescribed herein, to saturate the second absorbing medium with sulfurdioxide against a partial pressure of sulfur dioxide of no more thanabout 1 atmosphere at 25° C. The first and second absorbing media differfrom one another in water or type of amine absorbent so that the secondabsorbing has an absorption capacity for sulfur dioxide greater than thefirst absorbing medium. Any sulfur dioxide emissions produced in theapparatus 100 are removed therefrom and recycled via lines 102 and 76 tothe apparatus 78, in admixture with the sulfur dioxide-containing gasstream in line 76.

The second absorbing medium saturated with sulfur dioxide is withdrawnfrom the apparatus 100 and passed via line 104 to a buffer storagecontainer 106 from which it can be transferred via line 108 to atransport container (not shown) for transportation to a consuming site.Use can be made at the consuming site of the stripping column 56 shownin FIG. 4 for removing the absorbed sulfur dioxide from theSO₂-saturated absorbing medium to regenerate the diamine absorbentcontained in the second absorbing medium and thereby produce anothersulfur dioxide-depleted absorbing medium and a gaseous stream ofsubstantially pure water-saturated sulfur dioxide for consumption. Thesulfur dioxide-depleted absorbing medium which is recovered from thestripping column 56 is recycled to the apparatus 100 and fed therein vialine 110, for absorption of sulfur dioxide.

In the embodiment illustrated in FIG. 3, a sulfur dioxide-containing gasstream containing less than about 90% by volume of sulfur dioxide is fedvia line 112 to a gas-liquid contact apparatus 114 provided with achimney tray 116 defining a lower gas-liquid contact zone 118 and anupper gas-liquid contact zone 120 in gas flow communication with oneanother. The sulfur dioxide-containing gas stream in line 112 iscontacted in the lower zone 118 with a first absorbing medium comprisingwater and a diamine absorbent of the class described herein, to producea first sulfur dioxide-laden absorbing medium and a partially scrubbedsulfur dioxide-containing gas stream which flows upwardly throughchimney tray 116 and into the upper gas-liquid contact zone where it iscontacted with a second absorbing medium comprising water and a diamineabsorbent of the class described herein to produce a second sulfurdioxide-laden absorbing medium. The first and second absorbing mediadiffer from one another in water content or type of amine absorbent sothat the second absorbing medium has an absorption capacity for sulfurdioxide less than the first absorbing medium. Feed gas residue isdischarged from the apparatus 114 and passed via line 122 to a stack(not shown). The first sulfur dioxide-laden absorbing medium iswithdrawn from the apparatus 114 and passed via line 124 to a gas-liquidcontact apparatus 126. The second sulfur dioxide-laden absorbing mediumis also withdrawn from the apparatus 114 and passed via line 128 to astripping column 130 for removing the absorbed sulfur dioxide from thesecond absorbing medium and regenerating the diamine absorbent containedtherein, by means of steam fed from a reboiler 132 via line 134. Afraction of the first sulfur dioxide-laden absorbing medium mayoptionally be introduced via line 125 into the second sulfurdioxide-laden absorbing medium, for regenerating the diamine absorbentcontained therein. Steam is introduced into the reboiler 132 by line 136and condensate is removed therefrom by line 138. The sulfurdioxide-depleted absorbing medium is recovered from the stripping column130 by line 140, passed through the reboiler 132 and recycled via line142 to the second zone 120 of the apparatus 114 for absorption of sulfurdioxide. A fraction of the sulfur dioxide-depleted absorbing mediumrecovered from the stripping column 120 by line 140 and having a higherwater content than the absorbing medium recycled via line 142 is admixedwith the latter via line 143. A gaseous stream of substantially purewater-saturated sulfur dioxide is withdrawn from the stripping column130 into an overhead reflux condenser (not shown) and passed via line144 to the gas-liquid contact apparatus 126 wherein it is contacted withthe first sulfur dioxide-laded absorbing medium fed via line 124, tosaturate the first absorbing medium with sulfur dioxide against apartial pressure of sulfur dioxide of no more than about 1 atmosphere at25° C. Any sulfur dioxide emissions produced in the apparatus 126 areremoved therefrom and recycled via lines 146 and 112 to the lower zone118 of the apparatus 114, in admixture with the sulfurdioxide-containing gas stream in line 112.

The first absorbing medium saturated with sulfur dioxide is withdrawnfrom the apparatus 126 and passed via line 148 to a buffer storagecontainer 150 from which it can be transferred via line 152 to atransport container (not shown) for transportation to a consuming site.Use can be made at the consuming site of the stripping column 56 shownin FIG. 4 for removing the absorbed sulfur dioxide from theSO₂-saturated absorbing medium to regenerate the diamine absorbentcontained in the first absorbing medium and thereby produce anothersulfur dioxide-depleted absorbing medium and a gaseous stream ofsubstantially pure water-saturated sulfur dioxide for consumption. Thesulfur dioxide-depleted absorbing medium which is recovered from thestripping column 56 is recycled to the zone 118 of the apparatus 116 andfed therein via line 154, for absorption of sulfur dioxide. A fractionof the sulfur dioxide-depleted absorbing medium recycled via line 142 tozone 120 of the apparatus 114 may optionally be introduced via line 156into the sulfur dioxide-depleted absorbing medium fed via line 154 tozone 118 of the apparatus. The amount of diamine introduced via line 156serves to replace at least partially the amount of diamine contained inthe fraction of sulfur dioxide-laden absorbing medium passing in line125.

In the stripping columns 36, 56, 84 and 130, the operating pressure isgenerally in the range of 10-20 psi absolute.

While the above description of the preferred embodiments does notdescribe in detail all the equipment, such as heat exchangers, pumps,instrumentation necessary or desirable to the process, such equipmentwould be known to those skilled in the art.

The following diamines were evaluated for their effectiveness inabsorbing sulfur dioxide. The results are shown in Table 1:

TABLE I SO₂ Solubility, SO₂ Recovery, grains/100 grams of grams/100grams of Absorbent absorbent saturated absorbent 25% aqueous 32.4 13.9HEP* 45% aqueous 53.2 21.2 HEP* 25% aqueous 25.2 13.6 DIHEP** *HEP:N-(2-hydroxyethyl)piperazine (MW 130); **DIHEP:N,N′-bis(2-hydroxyethyl)piperazine (MW 174).

What is claimed is:
 1. A method of confining sulfur dioxide for storageor transportation under safe conditions, which comprises the steps of:a) contacting a sulfur dioxide-containing gas stream with an absorbingmedium comprising water and a water-soluble amine absorbent having atleast one amine group with a pKa value greater than about 7 and at leastone other amine group with a pKa value less than about 6.5 so that saidat least one amine group with a pKa value greater than about 7irreversibly absorbs sulfur dioxide in salt form rendering said amineabsorbent non-volatile and said at least one other amine group with apKa value less than about 6.5 reversibly absorbs sulfur dioxide, tothereby saturate said absorbing medium with sulfur dioxide against apartial pressure of sulfur dioxide of no more than about 1 atmosphere at25° C.; and b) charging the absorbing medium saturated with sulfurdioxide obtained in step (a) into storage or transportation means.
 2. Amethod as claimed in claim 1, wherein said amine absorbent has at leastone amine group with a pKa value of about 7.5 to about 10 and at leastone other amine group with a pKa value of about 4.5 to about 6.0.
 3. Amethod as claimed in claim 2, wherein said amine absorbent is a diamine.4. A method as claimed in claim 3, wherein said diamine has the generalformula:

wherein R₁ is an alkylene group having 1 to 3 carbon atoms, R₂, R₃, R₄and R₅ are the same or different and each represent a hydrogen atom, alower alkyl group having 1 to 8 carbon atoms or a lower hydroxy-alkylgroup having 2 to 8 carbon atoms, or any of R₂, R₃, R₄ and R₅ formtogether with the nitrogen atoms to which they are attached a 6-memberedring.
 5. A method as claimed in claim 4, wherein said diamine isselected from the group consisting ofN,N′,N′-(trimethyl)-N-(2-hydroxyethyl)-ethylenediamine,N,N,N′,N′-tetramethyl-ethylenediamine,N,N,N′,N′-tetramethyldi-aminomethane,N,N,N′,N′-tetrakis-(2-hydroxyethyl)-ethylenediamine,N,N′-dimethylpiperazine,N,N,N′,N′-tetrakis-(2-hydroxyethyl)-1,3-diaminopropane,N′,N′-di-methyl-N,N-bis-(2-hydroxyethyl)-ethylenediamine, N-methylN′-(2-hydroxyethyl)-piperazine, N-(2-hydroxyethyl)-piperazine,N,N′-bis(2-hydroxyethyl)-piperazine, N-methyl-piperazine, andpiperazine.
 6. A method as claimed in claim 5, wherein said diamine isN-(2-hydroxyethyl)-piperazine or N,N′-bis(2-hydroxyethyl)-piperazine. 7.A method as claimed in claim 1, wherein step (a) is carried out in agas-liquid contact apparatus providing countercurrent gas and liquidflows.
 8. A method as claimed in claim 1, wherein said sulfurdioxide-containing gas stream is a gaseous stream of substantially puresulfur dioxide.
 9. A method as claimed in claim 8, wherein step (a) iscarried out under substantially atmospheric pressure and ambienttemperature conditions.
 10. A method as claimed in claim 1, wherein saidsulfur dioxide-containing gas stream contains less than about 90% byvolume of sulfur dioxide, and wherein step (a) is carried out by: i)contacting said sulfur dioxide-containing gas stream with said absorbingmedium to produce a sulfur dioxide-laden absorbing medium; ii) dividingthe sulfur dioxide-laden absorbing medium into separate first and secondportions; iii) removing the absorbed sulfur dioxide from the secondportion of sulfur dioxide-laden absorbing medium to regenerate the amineabsorbent contained therein and thereby produce a sulfurdioxide-depleted absorbing medium and a gaseous stream of substantiallypure water-saturated sulfur dioxide; and iv) contacting the gaseousstream of substantially pure water-saturated sulfur dioxide with thefirst portion of sulfur dioxide-laden absorbing medium, whereby theproportion of sulfur dioxide-laden absorbing medium represented by saidsecond portion is such to produce in step (iii) sufficient gaseoussulfur dioxide to saturate in step (iv) said first portion of sulfurdioxide-laden absorbing medium with sulfur dioxide against a partialpressure of sulfur dioxide of no more than about 1 atmosphere at 25° C.11. A method as claimed in claim 10, wherein the sulfur dioxide-depletedabsorbing medium produced in step (a) (iii) is recycled to step (a) (i)for absorption of sulfur dioxide.
 12. A method as claimed in claim 10,wherein any sulfur dioxide emissions produced in step (a) (iv) arerecycled to step (a) (i) for admixture with said sulfurdioxide-containing gas stream.
 13. A method as claimed in claim 10,wherein the absorbing medium saturated with sulfur dioxide is chargedinto a transport container and transported to a consuming site where theabsorbed sulfur dioxide is removed from the saturated absorbing mediumto regenerate the amine absorbent contained therein and thereby produceanother sulfur dioxide-depleted absorbing medium and a gaseous stream ofsubstantially pure water-saturated sulfur dioxide for consumption.
 14. Amethod as claimed in claim 13, wherein said other sulfurdioxide-depleted absorbing medium is combined with the sulfurdioxide-depleted absorbing medium produced in step (a) (iii) and whereinthe combined sulfur dioxide-depleted absorbing media are recycled tostep (a) (i) for absorption of sulfur dioxide.
 15. A method as claimedin claim 13, wherein step (a) (i) is carried out in a gas-liquid contactapparatus providing countercurrent gas and liquid flows and having afirst inlet means for feeding said sulfur dioxide-containing gas streaminto said apparatus, a first outlet means for discharging feed gasresidue from said apparatus, a second inlet means adjacent said firstoutlet means for feeding the sulfur dioxide-depleted absorbing mediumproduced in step (a) (iii), a third inlet means between said first andsecond inlet means for feeding said other sulfur dioxide-depletedabsorbing medium so as to mix with the sulfur dioxide-depleted absorbingmedium fed through said second inlet means, in a mixing zone adjacentsaid third inlet means, and a second outlet means for discharging saidsulfur dioxide-laden absorbing medium.
 16. A method as claimed in claim1, wherein said sulfur dioxide-containing gas stream contains less thanabout 90% by volume of sulfur dioxide, and wherein step (a) is carriedout by: i) contacting said sulfur dioxide-containing gas stream with afirst absorbing medium comprising water and said amine absorbent toproduce a sulfur dioxide-laden absorbing medium; ii) removing theabsorbed sulfur dioxide from said sulfur dioxide-laden absorbing mediumto regenerate the amine absorbent contained therein and thereby producea sulfur dioxide depleted absorbing medium and a gaseous stream ofsubstantially pure water-saturated sulfur dioxide; and iii) contactingthe gaseous stream of substantially pure water-saturated sulfur dioxidewith a second absorbing medium comprising water and said amineabsorbent, said first and second absorbing media differing from oneanother in water content or type of amine absorbent so that said secondabsorbing medium has an absorption capacity for sulfur dioxide greaterthan said first absorbing medium, to saturate said second absorbingmedium with sulfur dioxide against a partial pressure of sulfur dioxideof no more than about 1 atmosphere at 25° C.
 17. A method as claimed inclaim 16, wherein the sulfur dioxide-depleted absorbing medium producedin step (a) (ii) is recycled to step (a) (i) for absorption of sulfurdioxide.
 18. A method as claimed in claim 16, wherein any sulfur dioxideemissions produced in step (a) (iii) are recycled to step (a) (i) foradmixture with said sulfur dioxide containing gas stream.
 19. A methodas claimed in claim 16, wherein said second absorbing medium saturatedwith sulfur dioxide is charged into said transportation means andconveyed to a consuming site where the absorbed sulfur dioxide isremoved from the saturated absorbing medium to regenerate the amineabsorbent contained therein and thereby produce another sulfurdioxide-depleted absorbing medium and a gaseous stream of substantiallypure water-saturated sulfur dioxide for consumption.
 20. A method asclaimed in claim 19, wherein said other sulfur dioxide-depletedabsorbing medium is recycled to step (a) (iii) for absorption of sulfurdioxide.
 21. A method as claimed in claim 1, wherein said sulfurdioxide-containing gas stream contains less than about 90% by volume ofsulfur dioxide, and wherein step (a) is carried out by: i) contactingsaid sulfur dioxide-containing gas stream with a first absorbing mediumcomprising water and said amine absorbent to produce a first sulfurdioxide-laden absorbing medium and a partially scrubbed sulfurdioxide-containing gas stream; ii) contacting said partially scrubbedsulfur dioxide-containing gas stream with a second absorbing mediumcomprising water and said amine absorbent, said first and secondabsorbing media differing from one another in water content or type ofamine absorbent so that said second absorbing medium has an absorptioncapacity for sulfur dioxide less than said first absorbing medium, toproduce a second sulfur dioxide-laden absorbing medium; iii) removingthe absorbed sulfur dioxide from said second sulfur dioxide-ladenabsorbing medium to regenerate the amine absorbent contained therein andthereby produce a sulfur dioxide-depleted absorbing medium and a gaseousstream of substantially pure water-saturated sulfur dioxide; and iv)contacting the gaseous stream of substantially pure water-saturatedsulfur dioxide with said first sulfur dioxide-laden absorbing medium tosaturate said first absorbing medium with sulfur dioxide against apartial pressure of sulfur dioxide of no more than about 1 atmosphere at25° C.
 22. A method as claimed in claim 21, wherein the sulfurdioxide-depleted absorbing medium produced in step (a) (iii) is recycledto step (a) (ii) for absorption of sulfur dioxide.
 23. A method asclaimed in claim 21, wherein any sulfur dioxide emissions produced instep (a) (iv) are recycled to step (a) (i) for admixture with saidsulfur dioxide-containing gas stream.
 24. A method as claimed in claim21, wherein said first absorbing medium saturated with sulfur dioxide ischarged into said transportation means and conveyed to a consuming sitewhere the absorbed sulfur dioxide is removed from the saturatedabsorbing medium to regenerate the amine absorbent contained therein andthereby produce another sulfur dioxide-depleted absorbing medium and agaseous stream of substantially pure water-saturated sulfur dioxide forconsumption.
 25. A method as claimed in claim 24, wherein said othersulfur dioxide-depleted absorbing medium is recycled to step (a) (i) forabsorption of sulfur dioxide.
 26. A method as claimed in claim 21,wherein steps (a) (i) and (a) (ii) are carried out in a gas-liquidcontact apparatus comprising first and second gas-liquid contact zonesin gas flow communication with one another, and wherein said sulfurdioxide-containing gas stream is contacted in said first zone with saidfirst absorbing medium, said partially scrubbed sulfurdioxide-containing gas flowing from said first zone to said second zonefor contact with said second absorbing medium in said second zone.
 27. Amethod as claimed in claim 26, wherein the sulfur dioxide-depletedabsorbing medium produced in step (a) (iii) is recycled to step (a) (ii)for absorption of sulfur dioxide in said second zone.
 28. A method asclaimed in claim 26, wherein any sulfur dioxide emissions produced instep (a) (iv) are recycled to step (a) (i) for admixture with saidsulfur dioxide-containing gas stream.
 29. A method as claimed in claim26, wherein said first absorbing medium saturated with sulfur dioxide ischarged into said transportation means and transported to a consumingsite where the absorbed sulfur dioxide is removed from the saturatedabsorbing medium to regenerate the amine absorbent contained therein andthereby produce another sulfur dioxide-depleted absorbing medium and agaseous stream of substantially pure water-saturated sulfur dioxide forconsumption.
 30. A method as claimed in claim 29, wherein said othersulfur dioxide-depleted absorbing medium is recycled to step (a) (i) forabsorption of sulfur dioxide in said first zone.